Golf balls have traditionally incorporated covers which are formed from polymeric materials. In this regard, the traditional golf ball having a cover formed from Balata rubber should be noted. The physical properties, and in particular, the "feel" of this Balata cover when struck with a golf club became the standard of the industry. In recent years golf balls have incorporated covers which are formed from synthetic polymeric materials such as polyolefins and in particular, polyethylene, polyurethanes and ionic copolymers of olefins. The latter mentioned ionic copolymers of olefins were commercially introduced in the mid 1960's by E. I. Du Pont De Nemours & Co., Inc., Wilmington, Del. and sold under the trademark "Surlyn." Cover compositions which are based on Surlyn resins have become extremely popular and are highly advantageous in that the resulting covers are extremely cut and abrasion resistant. Golf balls incorporating Surlyn resin covers are commonly known in golfing circles to be more cut resistant than Balata covered balls. These Surlyn covered golf balls have achieved widespread consumer acceptance. About sixty percent of the golf balls sold in the United States during 1976 incorporated Surlyn covers. Golf balls incorporating Surlyn covers are generally described in U.S. Pat. No. 3,454,280 issued July 8, 1969.
Most Surlyn resins as sold by E. I. De Pont De Nemours & Co., Inc., contain zinc or sodium ions. A plurality of Surlyn resins of varying physical properties are sold by E. I. De Pont De Nemours & Co., Inc. The physical properties of these various resins are described in technical bulletins which are readily available from E. I. De Pont De Nemours & Co., Inc. The principal Surlyn resins which are useful in this invention as golf ball cover materials are Surlyn 1605 and Surlyn 1557.
Mixtures of various Surlyn resins as cover stock materials are likewise highly advantageous. Suitable mixtures for use as cover stock materials are described in detail in U.S. Pat. No. 3,819,768 issued June 25, 1974.
Whether used singly or in mixtures, Surlyn resins, while being extremely advantageous due to their cut and abrasion resistance have one notable disadvantage as a golf ball cover stock material, namely that the feel of the ball when struck by a golf club is significantly different than that of a traditional Balata covered golf ball.
When a golf ball is struck by a golf club, an audible sound is emitted. This sound is generally referred to as the click of a golf ball. Again, the click of Surlyn covered golf balls is significantly different than the click of Balata covered golf balls. Some golfers and in particular, the purist who has traditionally used a Balata covered golf ball, find the change in click and the different feel of Surlyn covered golf balls to be objectionable.
For purposes of control some advanced golfers can strike a golf ball in such a manner that the flight of the ball incorporates substantial back spin. It is desirable that a golfer be able to impart back spin to a golf ball for purposes of controlling its flight and controlling the action of the ball upon termination of flight. For example, if back spin is incorporated into the flight of a ball, the ball can be made to stop dead once it strikes the green instead of bounding forward. The ability to incorporate back spin onto a golf ball is related to the extent to which a golf ball deforms when it is struck with a golf club. Because traditional Balata covers are more deformable it is easier to impart spin to these traditional balls. In contrast, Surlyn covered golf balls due to the nature of the cover are not as deformable as Balata covered balls and hence it has been more difficult to impart a sufficient degree of back spin to these balls. This control advantage will be discussed in greater detail herein below.
A standard which is generally referred to as the coefficient of restitution is one of the criteria useful in judging the rebound characteristics of a golf ball. The nature of the cover stock has substantial bearing on the coefficient of restitution of any given golf ball. By use of the subject invention one skilled in the art can alter the coefficient of restitution of a given golf ball to a much greater degree than was possible utilizing the covers of the prior art. The ramifications of the coefficient of restitution in relation to the subject invention will be described in greater detail herein below.
Competitive advertising campaigns comparing Balata and Surlyn resin covered golf balls have extensively exploited the above enumerated disadvantages of Surlyn resin covered golf balls. In particular, these advertising campaigns have exploited the different feel, and the difficulty of imparting spin to Surlyn resin covered golf balls. One advertising campaign even went so far as to compare a Surlyn resin covered golf ball to a rock. By incorporation of the subject invention into the golf ball structure it is possible for the first time to achieve and selectively alter the cut resistance, the abrasion resistance, the coefficient of restitution, the feel, the click and the ability to impart back spin to a golf ball. Generally speaking, by use of this invention, these physical characteristics of a golf ball can be varied to an optimum degree by varying the apparent density and distribution of the cells in a golf ball cover. As a result of these variations, one skilled in the art can produce a golf ball having the desirable qualities of both Balata and Surlyn resin covered golf balls.